PROJECT SUMMARY/ABSTRACT Atrial fibrillation (AF) is the most common sustained arrhythmia in adults, and leads to significant morbidity and mortality. Among the sickest patients with heart failure (HF), AF prevalence is up to 50%, and the prognosis of patients with both AF and HF is very poor. However, the burden of AF on HF patient reported outcomes (PROs) is not well-described. Additionally, while catheter ablation for AF is associated with improved overall outcomes, not all patients benefit from ablation. My goal in seeking a Mentored Research Career Development Award is to acquire the necessary training and experience to develop an independent research career focused on improving personalization of treatment and outcomes for patients with heart rhythm disorders. In this proposal we will close knowledge gaps in the management of patients with HF and AF, through the 3 specific aims, which are to: (1) compare PROs between HF patients with and without co-existing AF; (2) describe the clinical course of patients with AF and HF undergoing AF ablation; and (3) identify patients with AF and HF most likely to benefit from ablation for AF. Leveraging resources unique to the University of Utah, the proposed aims will provide novel insights into the burden of co-occurring HF and AF on quality of life using objective, structured, and systematic collection of validated PROs measures across the healthcare system. This career development plan will be mentored by internationally-renowned experts in the fields of health services research, PROs, outcomes in AF, HF therapies, and AF ablation: Dr. Rachel Hess, Chief of Health System Innovation and Research and Medical Director of the Utah PRO program; Drs. James Fang and Josef Stehlik, thought-leaders in HF; Drs. Jonathan Piccini and Nassir Marrouche, electrophysiologists and international leaders in AF ablation; Dr. John Spertus, a pioneer in PRO research; and Professor Tom Greene, an experienced statistical director. Combined with formal didactics, they will provide the support needed to achieve my training aims, developing skills in: (1) the use and interpretation of PROs; (2) analyses of longitudinal outcomes; and (3) the use of multivariable predictive modeling of treatment effect heterogeneity. In summary, the results of the proposed scientific aims will help guide the treatment of patients with HF and AF. With completion of the training aims, I will be uniquely-positioned as an invasive, clinical, cardiac electrophysiologist, educated in rigorous observational research methods with an emphasis on PROs and heterogeneity of treatment effects. The proposed Career Development Grant proposal will provide me the necessary skills to successfully pursue my goal of establishing an independent research program geared towards personalizing treatments for patients with heart rhythm disorders.